The following description relates to lateral bipolar transistors with regions of emitter, base and collector, which are arranged at a surface of a semiconductor device. The regions of emitter and collector extend vertically into semiconductor material and are doped for a first type of electric conductivity. Between the doped regions of emitter and collector, there is an oppositely doped base region at the surface. The base region forms pn junctions with both the emitter region and the collector region. Electric contacts of the emitter, collector and base can be arranged on the upper surface of the semiconductor device.
In automotive applications, for instance, bipolar transistors are applied to control voltages. For example, pnp transistors may be used. Bipolar transistors can also be applied as switches. In an H bridge, for example, there are four bipolar transistors arranged in two pairs, each pair having two bipolar transistors that are connected in series between the operating voltage of typically 12 V and ground. The connections between both transistors of each pair are connected to the driving connections of an electric motor, thus forming an electric circuit in the shape of an H, the motor being located in the position of the horizontal bar of the H. Such an H bridge enables the operation of the motor in any direction of rotation by simultaneously switching the diagonally arranged transistors on and off. This is also an application in which lateral pnp bipolar transistors are favorable.
Lateral bipolar transistors are characterized by a decrease of the amplification that is effected by the transistor structure with increasing distance from the device surface. This is due to the doping profile of the emitter and collector regions, which are implanted from the upper surface of the device into the semiconductor material. The doping profile results in a larger distance between emitter and collector in deeper regions of the semiconductor body. There is also an increasing electric resistance due to the larger distance from the surface contact.